Strap yourself in for a bumpy ride.
I had intermittent issues on my Sky line. At its worst there were 158 dropouts in 24 hours, at its best I went about three days without a drop on two occasions over a few month period.
Because there wasn't a "fault" when the Openreach engineer was in my house or at the cab, I kept being told there was little they could do.... I had to force them to actually look at the logs I'd collected via RouterStatsLite and the syslog - even though I'd pushed them to easy consume graphs and tables using Splunk.
I had 4 engineer visits without coming into the property, and 6 in the house... REIN checked extensively inside and outside the house and all the way back to the cab, lift and shift including D side change eventually done (visit to the house number 5, but 4 engineers had wanted to do it before that but the DCO said no).
After engineer visit 6 to my home, the problem still wasn't sorted and the engineer wanted to do a cease and re-provide, but the DCO said no. I then decided to leave Sky and go with Zen and bolt on Critical Care (I think Openreach call it Enhanced Care, but not all providers offer it).
The problem with intermittent issues is that the Openreach DCO processes don't allow them to authorise anything to be done if the fault's not there when they are. The first thing an engineer does when they arrive is unplug everything and plug in their own test equipment (understandably) - they then plug and unplug several pieces of equipment, which could all cause a drop to be missed even if it occurs whilst they're there....
Eventually my problem was solved. It disappeared without rhyme or reason several weeks after the last engineer visit but whilst I was waiting for the Zen line to go in....
VM Cable 152/12 + Zen 80/20 (+ Sky 80/20, about to cease)